Vehicle suspensions are characterized in various ways, including the stiffness in roll, single axle articulation, and cross articulation. There are two aspects of a suspension which determine how easily the wheels can move in articulation. One is the damping rate of the dampers, and the other is the spring rate of the springs in the suspension. The term stiffness refers to the spring rate. Therefore the articulation stiffness of the front axle of a vehicle is expressed as a force (expressed as a moment or a linear force) per unit of articulation displacement of the front wheels (expressed as an angular articulation displacement or a linear difference in ride height). The cross articulation stiffness is defined as a force per unit of cross articulation displacement, expressed, for example, as a linear difference in average ride height between two pairs of diagonally opposite wheels.
When a vehicle is traveling over rough terrain it is desirable to allow a high degree of articulation of the wheels, that is vertical movement of the wheels on opposite sides of the vehicle in opposite directions. In particular a high degree of cross articulation is desirable, that is articulation of the front wheels in one direction and articulation of the rear wheels in the opposite direction. This enables the vehicle to maintain traction over highly uneven ground. However this can conflict with the need for firm roll control when the vehicle is traveling at higher speeds on a road, since roll can be considered as articulation of the front and rear axles in the same direction.
This conflict arises, for example, in interconnected fluid suspensions, such as interconnected air suspensions, where the level of interconnection between air springs on opposite sides of the vehicle can be selected to control the level of resistance to articulation.
For example, it is also known from U.S. Pat. No. 5,765,115 to provide an air suspension system in which the air springs of the two rear wheels are interconnected by a pipe, which can be closed and opened by a gate valve, and the air springs of the two front wheels are similarly interconnected. Closing the valves increases roll stiffness of the vehicle, and opening them increases the ease of articulation.
The same conflict arises with the use of anti-roll bars which need to be stiff to provide good roll control on road, but much less stiff to allow sufficient articulation off-road.
This problem has been addressed, for example, by the type of system disclosed in U.S. Pat. No. 4,796,911 which discloses a vehicle with a split anti-roll bar, with a hydraulic de-coupling device between the two halves. A rough road sensor detects when the vehicle is travelling over a rough road and, in response, the two halves of the anti-roll bar are de-coupled from each other so as to increase the ease of articulation. On smooth roads the two halves are coupled together again so as to increase roll stiffness.
A similar conflict arises with dampers which are usually required to provide a relatively high level of damping for on-road use, but a reduced level of damping to increase ease of articulation during off road use. It is well known to address this problem using switchable dampers which have a damping rate which can be varied to suit the current driving conditions.
Whilst these systems do address the problem, there is always a need for better control of such systems.